Due to better modularization of crosscutting concerns, the Aspect oriented programming approach enhances the quality of the system as it results in less complex and more readable implementation of the system. As the software applications grow in size and complexity, they require some kind of high level organization. For high level organization of software system, packages are required. A lot of work has been carried out for measuring cohesion in Aspect Oriented Systems (AOS) at class level but very less research has been done for designing package level cohesion metric. Package cohesion metrics plays an important role in analyzing quality of software at package level. According to object oriented design principle, a good software design must have high cohesion with high reusability. Thus a relationship must therefore exist between cohesion and reusability. Number of attempts has been made to evaluate effect of cohesion on external attributes but at class level only. Impact of package level cohesion metrics on reusability for AOS is not yet explored. (a) To implement the proposed package cohesion measure, PCohA, on AspectJ sample packages, (b) to theoretically validate the proposed measure and (c) to find the impact of package cohesion on measuring reusability for AOS. Theoretical validation has been done by proving its validity on four theorems given by Briand et al. For finding the impact of proposed measure on external attributes, correlation has been found between package cohesion, PCohA, and external attribute—reusability. After theoretical validation, it has been proved that the proposed measure is suitable for measuring cohesion at package level. Correlation between package cohesion metric (PCohA) and reusability is calculated by using Karl Pearson Product Moment correlation. The computed values show a strong positive relation between PCohA and Reusability. The proposed package cohesion measure is found to be a useful indicator of external quality factors such as reusability. The proposed metric is also established as a better predictor of code reusability than the existing cohesion measures. The work discussed in this paper can be used for designing high quality software by developing new package level metrics for other quality attributes such as maintainability, changeability etc. as a future work. © 2017, Springer Science+Business Media New York.